Re: Neutron Stars

Well, in real life, if you were close enough to a neutron star to see it, you'd probably be fried before you had much of a chance to appreciate the view. :)

Nonetheless, we can try to imagine how the surface of a NS might look, were we able to get close to it. Much of what I write will be based on pulsars, which are fairly young NSs.

First off, how close would one have to be to an NS in order to see it? The radius of an NS is only about 10 km. Suppose we want to be close enough that the NS is about as big as a full Moon or Sun (about 1/2 degree on the sky). Then we'd have to be within about 2500 km of the NS! (For reference, that's a little bit less than the distance between New York City and Houston, Texas.)

The surface of an NS is quite hot, probably like 100 000 degrees Kelvin or hotter. The surface will therefore appear white hot or even bluish-white, primarily because most of the energy the NS is emitting is in ultraviolet or X-ray colors to which the human eye is not sensitive. The light that we can see, from blue to red, will be dominated by the blue light, giving the white or bluish-white color.

Pulsars earned their name because, in almost all cases, we receive only pulsed emission from them. (That is, unlike the Sun and other stars that shine steadily, pulsars turn on and off, much like how a lighthouse beacon behaves.) At their great distances, we can detect only radio light emission from the vast majority of pulsars. Some pulsars, however, can be detected in the visible part of the spectrum that we can see. It might be that all pulsars produce some visible light, but just too faint to be seen from distant pulsars. Close to a pulsar, we might be able to see the beam of the pulsar's emission (i.e., the lighthouse-like beam). If so, we would see a narrow cone or beam of light extending out into space. (This cone or beam of light also ends being involved in producing lots of high-energy particles and high-energy radiation. These are what would be responsible for frying you.) Of course, that assumes that the pulsar is spinning slow enough for us to be able to watch it. Most pulsars complete a "leisurely" 1 spin every second; there are pulsars that spin around hundreds of times every second, though.

Very near the surface of a pulsar, there is in fact an atmosphere. It's only about 1 cm or so thick. However, that atmosphere is a very dense plasma. The result is that, if you were to shine a light on it, the light would quickly be absorbed. In fact, it wouldn't be too much different from shining a light on a vat of liquid iron or steel---there's lots of light coming out, but essentially none of it would be from the flashlight you aimed at it.

I'll repeat the warning at the beginning, though. This discussion has been for a pulsar. As pulsars age, they lose energy, and they spin slower and cool. A sufficiently old and cool pulsar might have very little atmosphere. In that case, the outer surface would be like a metal surface, and shining a light on it might produce a reflection. However, I don't think that the Universe is old enough for any pulsars formed to have cooled this much yet.

I hope by now that you think pulsars are neat objects. For more information, see the Princeton's Pulsar Group (and links within) for additional information about pulsars.